SIGMET and AIRMET Explained

Updated: 2026-09-27 · First published: 2026-09-27 · About this site

A METAR describes one aerodrome; a TAF forecasts one. Neither says anything about the 200 miles in between. That is what SIGMETs and AIRMETs are for: warnings issued for a whole flight information region about weather that is dangerous en route — thunderstorms, severe turbulence, icing, mountain waves, dust storms, volcanic ash. They are terse, they use their own location grammar, and on the map they are the polygons drawn over the region. This guide explains how to read them.

Who issues what

SIGMET (significant meteorological information) is issued by the meteorological watch office responsible for a flight information region (FIR), for phenomena hazardous to all aircraft. ICAO Annex 3 lists them: thunderstorms (obscured, embedded, frequent, squall line, with hail), tropical cyclone, severe turbulence, severe icing (including freezing rain), severe mountain wave, heavy dust storm or sandstorm, volcanic ash, and radioactive cloud. Validity is up to four hours (six for ash and cyclones). A SIGMET is identified by a letter and sequence number, and is cancelled explicitly when the hazard ends.

AIRMET covers phenomena below FL100 (FL150 in mountainous areas) that are hazardous mainly to low-level and light-aircraft flight: moderate turbulence, moderate icing, surface wind above 30 kt, widespread visibility below 5,000 m, widespread cloud with base below 1,000 ft, isolated or occasional CB or TCU, mountain obscuration.

The United States uses the same words with a different shape: SIGMETs for non-convective hazards, Convective SIGMETs issued hourly for the eastern, central and western regions, and G-AIRMETs for Sierra (IFR and mountain obscuration), Tango (turbulence and wind) and Zulu (icing and freezing level), issued as graphical products with 3-hour snapshots.

Anatomy of an ICAO SIGMET

LTAA SIGMET 3 VALID 271400/271800 LTAC-
LTAA ANKARA FIR SEV TURB FCST WI N4100 E03000 - N4100 E03600 -
N3800 E03600 - N3800 E03000 - N4100 E03000 FL180/300 MOV E 25KT NC=

LTAA SIGMET 3 — the Ankara FIR, third SIGMET of the day.
VALID 271400/271800 — from the 27th at 14:00 to 18:00 UTC.
LTAC- — issuing meteorological watch office.
SEV TURB — the phenomenon: severe turbulence. Others: OBSC TS (obscured thunderstorms), EMBD TS (embedded in other cloud), FRQ TS (frequent, little separation), SQL TS (squall line), SEV ICE (FZRA), SEV MTW (mountain wave), HVY DS/SS, VA CLD, TC.
FCST WI … — forecast (or OBS AT time) within the polygon. Coordinates are latitude/longitude pairs; the polygon closes back to the first point. Some SIGMETs use N OF N4100, E OF E03000, or a line APRX 50NM WID LINE BTN ….
FL180/300 — vertical extent, FL180 to FL300. Others: SFC/FL100, TOP FL350 (for CB, the top), ABV FL200.
MOV E 25KT — moving east at 25 kt. STNR for stationary.
NC — no change expected. INTSF intensifying, WKN weakening.

Reading it against your route

The two questions are where will the polygon be when I get there and does my level intersect it. A cell moving east at 25 kt moves 50 NM in two hours; if the SIGMET was issued at 14:00 and you cross the area at 16:00, shift the polygon 50 NM east before deciding. A SEV TURB SIGMET for FL180/300 is irrelevant to a flight at 6,000 ft and decisive for one at FL240; a SEV ICE SIGMET at SFC/FL100 is the reverse. On the map on this site the current SIGMETs and AIRMETs are drawn as polygons that can be toggled on and off, so the intersection with a planned track is visible without plotting coordinates. Compare it with the individual station reports inside the polygon — LTAC Ankara, LTAE Ankara Esenboğa — which will show whether the forecast hazard has reached the surface.

What they do not cover

Aerodrome conditions. A SIGMET for a squall line 100 NM away does not tell you the crosswind at your destination — the METAR does.
Moderate anything, in a SIGMET. Moderate icing or turbulence belongs in an AIRMET; if the FIR does not issue AIRMETs (many do not), the information is in the area forecast (GAMET) or nowhere.
The gap between issuances. A SIGMET is issued when the forecaster is sufficiently confident; a developing squall line may be twenty minutes old before the SIGMET is out. Between SIGMETs, the METARs along the line — a sudden 30 kt gust and a wind shift at one station after another — are the earliest warning, and the thunderstorm guide covers what those look like.

A word on volcanic ash

VA SIGMETs are the one class where the hazard is invisible and the consequence is engine failure. They are issued for up to six hours from Volcanic Ash Advisory Centre forecasts and describe the cloud by polygon and flight levels, often in several layers. Modern jet engines have flamed out in ash that no crew could see at night. Treat a VA SIGMET as a closed area, not as advice.

Frequently asked questions

What is the difference between a SIGMET and an AIRMET?

Both are en-route warnings for a flight information region rather than an aerodrome. A SIGMET warns of weather hazardous to all aircraft — thunderstorms, severe turbulence, severe icing, dust or sand storms, volcanic ash, tropical cyclones. An AIRMET warns of weather that is hazardous mainly to light aircraft and low-level flight — moderate icing or turbulence, widespread low cloud or visibility, mountain obscuration, surface winds above 30 kt.

How long is a SIGMET valid?

Up to four hours, or six for volcanic ash and tropical cyclone SIGMETs, and it is cancelled or reissued if the hazard changes. AIRMETs and US AIRMETs (G-AIRMETs) are typically valid for six hours.

What does OBS AT 1200Z WI N4530 E03015 – … mean?

The hazard was observed (OBS; FCST would mean forecast) at 12:00 UTC within (WI) the polygon defined by the listed coordinates. The polygon is the area of the warning; the level group that follows gives its vertical extent.

What do MOV NE 20KT and INTSF mean?

The phenomenon is moving north-east at 20 kt and is intensifying. WKN is weakening, NC is no change, STNR is stationary. Use the movement to work out where the polygon will be at your ETA, not where it was when issued.

Does a SIGMET cancel a VFR flight?

Not automatically — it is information, not a prohibition — but a SIGMET for embedded thunderstorms or severe icing along your route is the strongest signal aviation meteorology has that the route is unsuitable. Read it with the TAFs and METARs along the route, not instead of them.